The issue is similar to this bug that has been fixed:
https://code.djangoproject.com/ticket/24428 however doesn't seem to be due
to coercion. I've discovered this when trying to update a project from
django 2.2 to 3.2
test case to reproduce (in a new django project/app)
```
from django.db import models
from django.forms import ModelForm
class Vehicle(models.Model):
modifications = models.JSONField(default=list, blank=True, null=True)
class VehicleForm(ModelForm):
class Meta:
model = Vehicle
fields = ("modifications", )
def test_vehicle_form():
vehicle = Vehicle.objects.create()
assert vehicle.modifications == []
data = {"modifications": "[]"}
form = VehicleForm(data, instance=vehicle)
assert form.is_valid()
assert not form.has_changed()
```
I would expect that test to pass, but it doesn't. I'm not sure exactly
where the error lies, but I have discovered the following:
```
field.show_hidden_initial == True
field.to_python(hidden_widget.value_from_datadict(self.data, self.files,
initial_prefixed_name)) == None
```
which then fails the check in field.has_changed
```
initial_value = initial if initial is not None else ''
data_value = data if data is not None else ''
return initial_value != data_value
```
because `data == []`
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